text: In competition reactions, the kinetic isotope effect is calculated from isotopic product or remaining reactant ratios after the reaction, but these ratios depend strongly on the extent of completion of the reaction. Most commonly, the isotopic substrate will consist of molecules labeled in a specific position and their unlabeled, ordinary counterparts. It is also possible in case of C kinetic isotope effects, as well as similar cases, to simply rely on the natural abundance of the isotopic carbon for the kinetic isotope effect experiments, eliminating the need for isotopic labeling.
  The two isotopic substrates will react through the same mechanism, but at different rates. The ratio between the amounts of the two species in the reactants and the products will thus change gradually over the course of the reaction, and this gradual change can be treated in the following manner:
  Assume that two isotopic molecules, A and A, undergo irreversible competition reactions in the following manner:
  The kinetic isotope effect for this scenario is found to be:
  Where F and F refer to the fraction of conversions for the isotopic species A and A, respectively.
  Isotopic enrichment of the starting material can be calculated from the dependence of R/R on F for various kinetic isotope effects, yielding the following figure.  Because of the exponential dependence, even very low kinetic isotope effects lead to large changes in isotopic composition of the starting material at high conversions.
  When the products are followed, the kinetic isotope effect can be calculated using the products ratio R along with R as follows: